- JulyBest month
- 102 daysWorkable days/yr
- 5 moSeason length
- 18–39%Rain-day odds
- Boulder by month
The next 10 days in Boulder
GOOD — every label check passes MARGINAL — one soft fail NO — hard fail or several soft
The rules this check uses
These rows are what the Boulder strip checks hour by hour: consensus paint-can requirements, plus the low-temp-formula band the engine marks MARGINAL.
| Check | Threshold | Why it matters |
|---|---|---|
| Air temperature | 50–90°F (low-temp formulas from 35°F) | The film (or mix) chemistry runs on temperature — both while you work and for the first hours after. |
| Overnight low | ≥35°F during the first 24 h (≥40°F preferred) | Curing continues after dark; the first night can undo a perfect afternoon. |
| Dry before | ≤0.05" rain in the prior 12 h; watch back to 24 h | Checked backward from your start hour using the two look-back days in the forecast data. |
| Dry after | <0.05" rain for 24 h after | The engine sums forecast rain hour by hour through the cure window for Boulder. |
| Evening dew-point spread | ≥5°F from 6–11 pm | When air temperature meets the dew point, water condenses on your fresh work first. |
| Daytime humidity | ≤80% | Read as the daytime maximum, 10 a.m.–5 p.m.; within 3 points of the limit counts as marginal. |
| Wind | ≤15 mph (brush only up to 20 mph) | Wind dries the leading edge too fast and carries overspray. |
Always follow your product label — formulas vary. The table above is the typical range across major manufacturers, not a promise about your can.
Best months for exterior painting in Boulder
| Month | Avg high | Avg low | Rain odds/day | Workable days | |
|---|---|---|---|---|---|
| January | 47°F | 22°F | 19% | 0 | |
| February | 48°F | 22°F | 23% | 0 | |
| March | 57°F | 29°F | 26% | 0 | |
| April | 63°F | 34°F | 34% | 0 | |
| May | 71°F | 43°F | 39% | 14 | |
| June | 82°F | 51°F | 35% | 20 | |
| July | 88°F | 57°F | 34% | 20 | |
| August | 86°F | 56°F | 34% | 21 | |
| September | 79°F | 48°F | 26% | 22 | |
| October | 66°F | 37°F | 22% | 5 | |
| November | 54°F | 28°F | 20% | 0 | |
| December | 46°F | 21°F | 18% | 0 |
The working season runs May through September — about 103 workable days a year. The edges are night-limited: label rules want overnight lows of 40°F+, and Boulder's nights only average that from May to September. The Colorado table ranks every listed city by the same math.
Boulder has a real wet/dry rhythm: May brings rain on 39% of days versus 18% in December. When the calendar gives you a December-side window, the dry-before and dry-after rules nearly take care of themselves.
Related check: roof coating in Boulder — same 50–90°F chemistry, but roofs hit the dew point first and wind is a safety stop.
Source honesty: every monthly figure on this page is the 1991–2020 normal at Boulder, Co Us, 3.8 km from Boulder's center — your block's shade lines and wind exposure sit on top of that baseline. Scoring details are on the methodology page.
For contrast, Aurora, CO’s exterior painting calendar sits one stop back on this route, and exterior painting in Castle Rock, CO is next in line.
Boulder by the numbers
- Hottest month: July — 88°F average high, 0 days topping 90°F.
- The cold floor is December at 46°F afternoons and 21°F overnight.
- Measurable rain: May leads at 39% of days; December is the quiet end at 18%.
- The 40°F-night season spans May–September here.
- Bottom line for Boulder: roughly 103 workable exterior painting days a year.
Prep checklist
- Two clean days beat one perfect one: 24 h of dry cure and a 40°F+ night — September is Boulder's highest-odds month (22 days).
- Scrape, then wash: loose paint and chalk go first, because latex only grips solid substrate.
- Give washed siding 24 hours — north walls in Boulder can need double after a May-grade soak.
- Read the wall, not the app: an ir surface thermometer on sunlit siding shows 20°F+ over Boulder's reported 79°F.
- Bare wood gets primer, stains get stain-blocker, gaps get caulk — in that order, on dry substrate.
- Follow the shade around the house — never a wall in direct midday sun.
- Cut in with an angled brush set, roll a wet edge, and drop the sprayer plan over 15 mph.
- Stop 2 hours before sunset: with September lows near 48°F, Boulder's siding meets the dew point before the late news.
Pro moves for Boulder conditions
- Follow the shade around the house instead of the sun; siding that's hot to the touch will blister paint even when air temperature reads fine.
- Cold-rated acrylics that apply down to 35°F still need surfaces to stay above freezing for the next 36 to 48 hours, so check overnight lows.
- Dark colors can push a sunlit wall 30°F or more above air temperature; schedule dark accents for early morning or overcast days.
- Stop painting two to three hours before the evening dew point converges with air temperature, or the finish can develop dull spots and surfactant streaks.
- In Boulder, humidity typically bottoms out mid-afternoon; save the final topcoat for that window and use mornings for prep and priming.
Gear that saves a window
Transparency note: gear links here become affiliate links only when the program is enabled — today they are plain references. See the affiliate disclosure.
-
IR surface thermometer
Reads the wall, not the air — sun-baked siding runs hotter.
-
Extension pole
Second-story reach without moving the ladder every pass.
-
Exterior paint + primer
One-coat hide on sound, prepped siding.
-
Angled brush set
Control at trim, corners, and cut-in lines.
Frequently asked questions
What temperature can you paint outside?
Standard latex: 50–90°F with nights of 40°F+; low-temperature formulas are rated down to 35°F and the engine marks 35–50°F highs as MARGINAL for exactly that reason. Boulder's edge months live in that band — May averages 71°F highs over 43°F nights.
How many dry hours does exterior paint need before rain?
About 24 — a 0.05"+ shower inside that window streaks or washes fresh latex. Boulder offers those 24-hour dry runs most reliably in December (rain on just 18% of days); May is the gamble at 39%.
Why does dew ruin fresh paint?
Because a film that hasn't coalesced can't shed water: on cooling Boulder siding, dew flat-spots the sheen and drags surfactants out in streaks. It forms when the wall reaches the dew point — the engine wants a 5°F spread from 6–11 p.m. Finish 2 hours before sunset and latex gets its lead time.
Can you paint in high humidity?
The label limit is ~80% relative humidity, and it compounds: humid air slows the cure, which pushes wet film into dew hours. The engine flags 80–83% and fails beyond. In Boulder, the drier December air makes this a non-issue; muggy spells make it the day-killer.
What is surface temperature vs air temperature?
The forecast reports air; the label limits the wall. In direct sun a wall runs 20°F+ hotter — a 88°F Boulder July day can put a west wall past the 90°F ceiling by mid-afternoon. Follow the shade around the house and check the surface by hand or IR thermometer.
When does painting season end in Boulder?
The closing bell is the overnight floor. September is the last month averaging viable nights (48°F lows); after that, even warm afternoons sit on failing nights. Spring reopens around May from the same rule.
What humidity is too high for exterior painting?
Most acrylic paints apply well between 40 and 70 percent relative humidity. Above about 85 percent, water can't evaporate from the film, so drying stalls, sags develop, and surfactants rise as sticky brown streaks. Very low humidity under 20 percent causes the opposite problem — paint dries too fast to level. Check the hourly forecast for Boulder and aim for a falling-humidity afternoon.
What is the dew point rule for exterior painting?
Coating manufacturers require the surface to be at least 5°F above the dew point during application and initial drying. Closer than that, an invisible film of condensation forms on the siding, which ruins adhesion and can flash-rust bare metal. Practically, start after morning dew has fully evaporated and stop early enough that paint is set before the evening temperature drop.
How windy is too windy to paint outside?
Sustained winds above about 15 mph make brushing and rolling risky: paint skins before it levels, lap marks multiply, and dust or pollen embeds in the wet film. Spraying is far more sensitive — overspray drifts onto cars and neighbors above roughly 5 to 10 mph. Wind also accelerates drying, shortening your wet edge, so pick calm mornings for large walls.
Why is my fresh exterior paint blistering?
Heat blisters form when paint is applied to siding baking in direct sun — the surface skins over, then trapped solvent or water vaporizes underneath. Moisture blisters appear when paint goes over damp wood or when interior moisture pushes outward. Paint shaded sides, keep surfaces under about 90°F, and let rain-soaked siding dry a day or two before coating.
Exterior Painting glossary
- Dew point spread
- The gap between air temperature and dew point; painting standards call for surfaces at least 5°F above the dew point to prevent invisible condensation under fresh paint.
- Surfactant leaching
- Sticky brown or glossy streaks that surface when latex paint dries in cool, damp conditions; usually cosmetic and rinses off after the paint fully cures.
- Wet edge
- Keeping the boundary of just-applied paint wet so the next stroke blends invisibly; heat, wind, and low humidity shorten it and cause lap marks.
- Surface temperature
- The actual temperature of siding, which can run 20 to 40°F above air temperature in direct sun; paint labels' limits apply to the surface, not the forecast.
- Full cure
- The point when paint reaches final hardness and washability — typically 2 to 4 weeks for exterior acrylics — distinct from dry-to-touch, which takes only hours.
- Blocking
- Fresh paint sticking to itself where surfaces touch, like doors against jambs; worst in heat and humidity before the film fully cures.
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Guides
Climatology: NOAA 1991–2020 normals via BOULDER, CO US (3.8 km from Boulder center, elevation 5484 ft); live outlook by Open-Meteo.